Metal Finishing Guide Book


Issue link:

Contents of this Issue


Page 565 of 843

overspray, and you have already said you do not have good ventilation in the basement. WHICH PAINT SPRAY APPLICATION METHOD PRODUCES A DRIER COATING? Q: I was pondering over two issues: First, if all variables are held constant, and you apply an automotive waterborne paint, which application would produce a drier film (using an ESTA bell) one at 35,000 RPM or one at 55,000 RPM? Second, which application is drier: electrostatic or conventional? A: If all variables are held constant, I would imagine that the faster rotational speed of the electrostatic bell would break up the paint into finer particles, leading to a drier finish. As I have never performed this experiment, this is only a guess. I would imagine that the same applies to your second question: whichever method produces smaller particles would produce a drier finish. In this case, I presume the electrostatic spray gun may produce smaller particles, as it can be expected to accelerate evaporation of the solvent compared to a conventional process. CALCULATING AIR FLOW IN SPRAY BOOTHS Q: I have a product that I will be coating with Waterborne Camouflage Aliphatic Polyurethane Chemical Agent Resistant Coating (CARC). I will apply the coating with an HVLP sprayer at approximately 65 psi with a 0.070" tip. I calculated the sprayer volume flow rate to be about 4 SCFM. I am trying to calculate air flow needed for my spray booth and the corresponding energy consumption. If my makeup air is coming from outside, I have to condition the air for humidity and temperature prior to entering the paint booth. I estimate my booth will have a footprint of 100–150 ft2. There will be two operators in the booth: one doing prep; one painting. The product will be hanging on a conveyor at working height. I have read about cross-draft and down-draft booths with velocity requirements of 100 ft/min and 50 ft/min, respectively. Would one flow rate be preferred over the other for waterborne CARC? What is the main difference between the two flow rates? To calculate the volume flow rate needed, do I multiply the velocity spec by the area of the filter area? How do I know how much filter area I need? Can I bring make-up air into the booth from the main building in which the spray booth is located? Do I draw the make-up air into the booth with my exhaust fan or push it in with another fan? If I do have to bring in make-up air from outside, do you know a reputable company that manufactures efficient equipment for controlling the temperature and humidity? A: The air flow rate inside the booth is independent of the spray gun, its pressure, air throughput, etc. According to the National Fire Protection Association (NFPA) Bulletin 33, you need to move air through the booth at approximately 100 ft/min. You can calculate this by using the cross-sectional area of the booth. Therefore, if the width times the height of the spray booth equals 80 ft2 you would need an air flow rate of 8,000 ft3/min. If you wish to lower this, you need to calculate the lower explosive limit (LEL) of the WD-CARC, but this is not a simple calculation. With this information, you then need to calculate the air velocity that will maintain the concentration of solvents at 25% LEL plus a sufficient safety factor. Suppose you were to find 556

Articles in this issue

view archives of Metal Finishing Guide Book - 2013